Abstract
Cast austenitic stainless steel, such as SCS 13, which consists of δ/γ duplex phase, is extensively used as structural material in seawater environments. However, it undergoes pitting corrosion under certain conditions. In this paper, effects of δ ferrite on the pitting corrosion are discussed in terms of microstructure of the material. The corrosion behavior of the materials with different δ ferrite contents and cleanlinesses have been evaluated by seawater exposure testing, electrochemical polarization, and TEM-EDX analysis. The results indicate that pits mainly nucleate at nonmetallic inclusions such as MnS and δ/γ boundaries. The nucleation at the latter sites is assumed to be due to the segregation of P combined with Mo and Cr. The segregation is suppressed for materials containing δ ferrite above 6vol%, and such materials are highly resistant to pitting corrosion.
